This invention relates to lignocellulosic composites, and more particularly, to lignocellulosic, borate filled, thermoplastic composites.
There is a very high demand for wood products. Although wood is a renewable resource, it takes many years for trees to mature. Consequently, the supply of wood suitable for use in construction is decreasing and there is a need to develop alternatives.
Lignoelluosic materials, such as wood, sawdust, rice hulls, and the like have long been added to thermoplastic resins such as polyethylene, polypropylene and polyvinyl chlorine (PVC) to achieve a wood-like composite providing reinforcement, reduced coefficient of expansion, and cost reduction. Process methods have been developed to enable blends containing materials having low bulk density (ie. powders) and poor flow characteristics to be fed at commercially acceptable rates. Blends of this type can be extruded through dies of appropriate configuration to produce building product type shapes previously made from wood. When these thermoplastic composites were first introduced, the prevailing theory was that the plastic protected the cellulose from fungal attack. However research by Verhey, Laks, and Richer, described in “Laboratory Decay Resistance of Woodfiber/Thermoplastic Composites”, Forest Products Journal, September 2001 revealed that lignocellulosic thermoplastics are susceptible to damage from fungal decay. Degradation due to the fungal attack is a problem that threatens the material's structural integrity. In contrast, surface discoloration and spotting has been reported shortly after the introduction of thermoplastic composites. This visual degradation, caused by mold, is a significant problem since major commercial uses of lignocellulosic thermoplastic composites, including decking and fencing, rely on their aesthetic appeal to compete in the marketplace.
Traditionally, solid wood products are dipped or pressure treated with solutions of fungicides to provide resistance to fungus and mold damage. While this type of treatment is not practicable for a thermoplastic product, it is possible to incorporate a fungicide into the product during its manufacture. This approach provides a constant loading of fungicide throughout the material's thickness, increasing the resistance to leaching of the fungicide from the composite. However it diminishes surface concentration of the fungicide, reducing its effectiveness against surface mold attack. Anhydrous borate and zinc borate have been used successfully to provide fungal decay at relatively low levels, typically less than 1.5 percent, in lignocellulosic compounds formed from small fractions of wood bonded with an adhesive binder of phenol-formaldehyde resin as described in U.S. Pat. No. 4,879,083. Zinc borate has also been described in the literature as providing resistance to fungal decay in lignocellulosic filled thermoplastics. Research on zinc borate's use as an anti-fungal additive in lignocellulosic thermoplastics has focused on the minimum loading required to increase resistance to fungal decay, while neglecting to consider or investigate the effect of those higher loading levels required to provide resistance to visual deterioration caused by surface molds.
Although not used commercially as a fungicide, calcium borate is described in U.S. Pat. No. 6,368,529 and Patent Application No 20020182431 as providing protection against fungal decay and insects in lignocellulosic compounds formed from small fractions of wood bonded with an adhesive binders of phenol-formaldehyde, phenol-resorcinol-formaldehyde, urea-formaldehyde, and diphenylmethanediisocyanate at preferred levels of 1.5% to 15%. All investigation done on the use of calcium borate as a fungicide has focused on its ability to resist fungal decay in lignocellulosic composites such as particleboard, waferboard, oriented strandboard, and medium density fiberboard that use these thermosetting resins.
Currently the lignocellousic thermoplastics industry is faced with two preservation needs: (1) finding an economic method of improving resistance to fungal decay and (2) developing an economic method for improving resistance to the visual damage caused by surface mold.